Breech bolt for firearm

ABSTRACT

A firearm including a receiver, a barrel, a breech bolt assembly, and a trigger assembly. The bolt assembly has a bolt including a body portion, a lug portion, and a firing pin bore. The lug portion includes outward extending lugs, a cartridge recess, and an off-center hole. The bolt assembly also includes an extractor and an ejector pin. At least some of the lugs on the lug portion of the bolt have a tapering outer face so that a rearward face of the respective lug is taller than a forward face of the respective lug. In another aspect, at least some of the lugs on the lug portion of the bolt have tapering side faces so that a rearward face of the respective lug is wider than a forward face of the respective lug.

BACKGROUND

The present invention relates to firearm breech bolt assemblies, andmore particularly, but not exclusively, to a breech bolt having improvedreliability.

Automatic rifles have been standard weapons of choice for the armedforces and police SWAT units. These weapons also have semi-automaticcounterparts which are popular with civilians. Many of these automaticand semi-automatic firearms are based on a gas-operated breech boltcarrier system. U.S. Pat. Nos. 2,951,424 and 3,198,076 both to Stonerprovide early examples of these types of weapons. Generally, the boltcarrier system of these weapons includes a multi-lug breech bolt thatinterlocks with lugs on a bolt receiver for firing each round ofammunition. The pressurized gases resulting from the weapon firing abullet are directed to slide the breech bolt backward and then forwardin the receiver, ejecting a spent shell casing and loading a newcartridge from a magazine adjacent the receiver.

Occasionally, the breech bolt becomes misaligned with the receiver whenin the bolt is moving forward so the lugs on the breech bolt are nolonger aligned with gaps between the lugs on the receiver. When the lugsand gaps slots are out of alignment, the bolt cannot slide forward tochamber the new cartridge and the bolt becomes jammed. Jammed boltslimit the overall reliability of the weapon. By reducing the frequencyof jammed bolts, maintenance-actions for the gun are correspondinglyreduced and overall reliability is improved. Consequently, there is aneed to better ensure the bolt lugs properly mesh with the receivergaps.

SUMMARY

In one aspect, the present invention includes a firearm for firing aprojectile from a cartridge including a shell casing having a hollowinterior, propellant in the interior of the shell casing, and theprojectile mounted at a forward end of the shell casing. The firearmcomprises a receiver having a cavity therein and a barrel interface at aforward end thereof. The barrel interface includes a bolt interlockingchamber and a plurality of receiver lugs spaced by gaps and extendinginward behind the bolt interlocking chamber. The firearm has an elongatebarrel mounted on the barrel interface for directing the projectileforward when fired from the cartridge. The barrel includes a firingchamber at a rearward end for holding the cartridge prior to firing theprojectile and for holding the shell casing after the projectile isfired. The firing chamber is positioned in front of the boltinterlocking chamber. Further, the firearm includes a breech boltassembly mounted in the receiver cavity behind the barrel interface forloading the cartridge in the firing chamber from a cartridge sourceprior to firing, and extracting the shell casing from the firing chamberafter firing. The bolt assembly comprises a bolt including an elongatecylindrical body portion having a longitudinal centerline, a lug portionextending forward from the body portion, and a firing pin bore extendingthrough the bolt parallel to the longitudinal centerline. The lugportion includes a plurality of lugs extending outward therefrom, acartridge recess at a front end sized and shaped for holding a rearwardend of the cartridge therein, and an off-center longitudinal hole atleast partially aligned with the cartridge recess. The plurality of lugsis spaced to correspond to the gaps between the receiver lugs so thatthe plurality of bolt lugs can slide forward through the gaps and rotateabout the centerline of the body portion to align at least some of theplurality of bolt lugs with at least some of the plurality of receiverlugs. The bolt assembly also includes an extractor pivotally mounted inthe extractor cavity and biased to hold the cartridge in the cartridgerecess, and an ejector pin slidably received in the off-centerlongitudinal hole. The pin is biased to eject the shell casing from thecartridge recess. The firearm also has a trigger assembly including afiring pin slidably received in the firing pin bore of the bolt andaligned with the cartridge when received in the cartridge recess andfiring chamber for initiating firing of the projectile from thecartridge. At least some of the plurality of lugs on the lug portion ofthe bolt have a tapering outer face so that a rearward face of therespective lug is taller than a forward face of the respective lug.

In another aspect, the present invention includes a breech bolt assemblyfor mounting in a firearm for loading a cartridge in a firing chamber ofthe firearm prior to firing a projectile from the cartridge andextracting a shell casing from the firing chamber after firing. The boltassembly comprises a bolt including an elongate cylindrical body portionhaving a longitudinal centerline, a lug portion extending forward fromthe body portion, and a firing pin bore extending through the boltparallel to the longitudinal centerline. The lug portion includes aplurality of spaced lugs extending outward therefrom, a cartridge recessat a front end sized and shaped for holding a rearward end of thecartridge therein, and an off-center longitudinal hole at leastpartially aligned with the cartridge recess. The bolt assembly alsoincludes an extractor pivotally mounted in the extractor cavity andbiased to hold the cartridge in the cartridge recess, and an ejector pinslidably received in the off-center longitudinal hole. The pin is biasedto eject the shell casing from the cartridge recess. At least some ofthe plurality of lugs on the lug portion of the bolt have a taperingouter face so that a rearward face of the respective lug is taller thana forward face of the respective lug.

In still another aspect, the present invention includes a method forincreasing reliability of a firearm, comprising removing a used breechbolt from the firearm, and installing a new breech bolt in the firearm.The new bolt has a plurality of lugs have a tapering outer face so thata rearward face of each respective lug is taller than a forward face ofthe respective lug.

In yet another aspect, the present invention includes a method forincreasing reliability of a firearm, comprising removing a breech bolthaving a plurality of lugs from the firearm, and reworking the breechbolt so at least some of the lugs have a tapering outer face. The outerface tapers so that a rearward face of each respective lug is tallerthan a forward face of the respective lug. The method also includesinstalling the reworked breech bolt in the firearm.

In another aspect, the present invention includes a firearm for firing aprojectile from a cartridge including a shell casing having a hollowinterior, propellant in the interior of the shell casing, and theprojectile mounted at a forward end of the shell casing. The firearmcomprises a receiver having a cavity therein and a barrel interface at aforward end thereof. The barrel interface includes a bolt interlockingchamber and a plurality of receiver lugs spaced by gaps and extendinginward behind the bolt interlocking chamber. The firearm has an elongatebarrel mounted on the barrel interface for directing the projectileforward when fired from the cartridge. The barrel includes a firingchamber at a rearward end for holding the cartridge prior to firing theprojectile and for holding the shell casing after the projectile isfired. The firing chamber is positioned in front of the boltinterlocking chamber. Further, the firearm includes a breech boltassembly mounted in the receiver cavity behind the barrel interface forloading the cartridge in the firing chamber from a cartridge sourceprior to firing, and extracting the shell casing from the firing chamberafter firing. The bolt assembly comprises a bolt including an elongatecylindrical body portion having a longitudinal centerline, a lug portionextending forward from the body portion, and a firing pin bore extendingthrough the bolt parallel to the longitudinal centerline. The lugportion includes a plurality of lugs extending outward therefrom, acartridge recess at a front end sized and shaped for holding a rearwardend of the cartridge therein, and an off-center longitudinal hole atleast partially aligned with the cartridge recess. The plurality of lugsis spaced to correspond to the gaps between the receiver lugs so thatthe plurality of bolt lugs can slide forward through the gaps and rotateabout the centerline of the body portion to align at least some of theplurality of bolt lugs with at least some of the plurality of receiverlugs. The bolt assembly also includes an extractor pivotally mounted inthe extractor cavity and biased to hold the cartridge in the cartridgerecess, and an ejector pin slidably received in the off-centerlongitudinal hole. The pin is biased to eject the shell casing from thecartridge recess. The firearm also has a trigger assembly including afiring pin slidably received in the firing pin bore of the bolt andaligned with the cartridge when received in the cartridge recess andfiring chamber for initiating firing of the projectile from thecartridge. At least some of the plurality of lugs on the lug portion ofthe bolt have tapering side faces so that a rearward face of therespective lug is wider than a forward face of the respective lug.

In another aspect, the present invention includes a breech bolt assemblyfor mounting in a firearm for loading a cartridge in a firing chamber ofthe firearm prior to firing a projectile from the cartridge andextracting a shell casing from the firing chamber after firing. The boltassembly comprises a bolt including an elongate cylindrical body portionhaving a longitudinal centerline, a lug portion extending forward fromthe body portion, and a firing pin bore extending through the boltparallel to the longitudinal centerline. The lug portion includes aplurality of spaced lugs extending outward therefrom, a cartridge recessat a front end sized and shaped for holding a rearward end of thecartridge therein, and an off-center longitudinal hole at leastpartially aligned with the cartridge recess. The bolt assembly alsoincludes an extractor pivotally mounted in the extractor cavity andbiased to hold the cartridge in the cartridge recess, and an ejector pinslidably received in the off-center longitudinal hole. The pin is biasedto eject the shell casing from the cartridge recess. At least some ofthe plurality of lugs on the lug portion of the bolt have tapering sidefaces so that a rearward face of the respective lug is wider than aforward face of the respective lug.

In yet another aspect, the present invention includes a method forincreasing reliability of a firearm. The method comprises removing aused breech bolt from the firearm, and installing a new breech bolt inthe firearm. The new bolt includes a lug on the lug portion of the bolthave tapering side faces so that a rearward face of the respective lugis wider than a forward face of the respective lug.

And, in a still another aspect, the present invention includes a methodfor increasing reliability of a firearm. The method comprises removing abreech bolt having a plurality of lugs from the firearm, and reworkingthe breech bolt so a lug on the lug portion of the bolt has taperingside faces so that a rearward face of the respective lug is wider than aforward face of the lug. The method includes installing the reworkedbreech bolt in the firearm.

Other aspects of the present invention will be apparent in view of thefollowing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross section of a firearm of one embodiment ofthe present invention;

FIG. 2 is a side elevation of a breech bolt of the weapon depicted inFIG. 1;

FIG. 3 is a top plan of the breech bolt depicted in FIG. 2;

FIG. 4 is a front elevation of the breech bolt depicted in FIG. 2;

FIG. 5 is a cross section of the breech bolt taken along line 5-5 ofFIG. 2;

FIG. 6 is a cross section of a barrel interface of the embodimentdepicted in FIG. 1; and

FIG. 7 is a rear elevation of the barrel interface depicted in FIG. 6.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a firearm incorporating one embodiment of thepresent invention is designated in its entirety by the reference number20. The firearm 20 has a barrel 22 and a cartridge source such as amagazine 24. The magazine 24 is configured to sequentially feedcartridges to a firing chamber 26 through a receiver 30. The receiver 30includes a trigger assembly 32 with a spring-loaded trigger 34, aspring-loaded hammer 36, and a firing pin 38. The receiver 30 has acavity 40 configured to house a breech bolt assembly 42. The breech boltassembly 42 has a firing pin bore 44 through which the firing pin 38extends. The breech bolt assembly 42 also includes a spring-loadedextractor 46 and a breech bolt 50. The extractor 46 has a guide flange52 and is pivotably coupled by a pivot pin 54 to the breech bolt 50.

As illustrated in FIGS. 2 and 3, the breech bolt 50 has a cylindricalbody portion 56, as well as opposite front and back ends 58, 60,respectively, spaced along a longitudinal centerline L. A stem 62extends longitudinally from the back end of the body portion 56. Thestem 62 includes a circular sealing flange 64 having a circumferentialgroove 44 configured for receiving conventional sealing rings (notshown). A neck 68 separates the sealing flange 64 from the cylindricalbody portion 56. The body portion 56 has a longitudinal extractor cavity72 for receiving an extractor 46 and a lateral hole 76 intersecting theextractor cavity for receiving an extractor pivot pin 78 to hold theextractor in position in the cavity. A larger lateral hole 80 extendsacross the breech bolt body portion 56 behind the extractor 46 forreceiving a conventional cam pin (not shown) to rotate the bolt 50between an unlocked and locked position as is understood by thoseskilled in the art.

As shown in FIGS. 2-4, the breech bolt 50 also has lug portion 90 at thefront end of the cylindrical body portion 56. The lug portion 90includes a cartridge recess 92 to receive a rearward end of a chamberedcartridge C. The extractor 46 is biased radially inward by a coil spring(not shown) for holding the chambered cartridge C in the cartridgerecess 92. As illustrated in FIG. 4, the lug portion 90 also has anoff-center, longitudinal hole 96 that extends well into the cylindricalbody portion 56. The hole 96 receives an ejector pin 98 for ejecting aspent shell casing as is well known in the art. As shown in FIG. 2, thebody portion 56 includes a lateral hole 100 that intersects the hole 96for receiving a split pin (not shown) to slidably capture the ejectorpin 98 in the hole. A coil spring (not shown) is positioned between theejector pin 98 and a bottom of the hole 96 to bias the ejector pin inthe forward direction. The body portion 56 includes a radial hole (notshown) that intersects the hole 96 for confirming the coil spring is inposition between the pin 98 and the bottom of the hole.

The lug portion 90 has a plurality of bolt lugs 110 a-g, extendingradially about longitudinal axis L. An eighth lug 110 h extends radiallyfrom the extractor 46. Although this eighth lug 110 h does not functionhold the bolt 50 in position against recoil forces during cartridgefiring like the other lugs 110 a-g, it projects from the extractor 46and is intended to be included within the meaning of the term lug hasused herein. The lugs 110 a-h are evenly spaced about the bolt 50 atabout 45° intervals. Lugs 110 a-g have flat outer lands 112 a-g spaced acommon distance from the longitudinal axis L, but lug 110 h has a flatouter land 112 h spaced a shorter distance from the longitudinal axis(regardless of whether the extractor 46 is holding a cartridge). Lugs110 a-g have flat radially extending forward and rearward faces 114 a-g(FIG. 4), 116 a-g (FIG. 5), respectively, positioned at common positionsalong the longitudinal axis L. As shown in FIG. 3, lug 110 h, however,has radially skewed forward and rearward faces 114 h, 116 h,respectively. As will be appreciated by those skilled in the art, therearward faces 116 a-g of lugs 110 a-g are load-bearing surfaces duringcartridge firing.

As further illustrated in FIG. 4, unlike conventional bolts, each of thelugs 110 a-d&g of the bolt 50 of the illustrated embodiment has arounded tapering outer face 120 a-d and 120 g extending from itsrespective outer land 112 a-d and 112 g to its respective forward face114 a-d and 114 g. Although the tapering outer faces 120 a-d and 120 gmay have other dimensional characteristics without departing from thescope of the present invention, in one embodiment the faces are taperedat an angle in a range of about 5° to about 40° measured fromlongitudinal, more particularly at an angle in a range of about 10° toabout 32° measured from longitudinal, and still more particularly at anangle of about 21°. Although the forward faces 114 a-d and 114 g mayhave other dimensional characteristics without departing from the scopeof the present invention, in one embodiment the faces are shorter thanthe forward faces 114 e and 114 f by a radial distance in a range ofabout 0.010 in. to about 0.060 in., more particularly by a radialdistance in a range of about 0.025 in. to about 0.045 in., and stillmore particularly by a radial distance of about 0.035 in.

Further, as illustrated in FIG. 5, each of the lugs 110 a-h havecircumferential side faces 122 a-h and 124 a-h, extending longitudinallyalong the bolt 50. Although each of the side face pairs (i.e., 122 a,124 a; 122 b, 124 b; etc.) may taper toward each other, forming varyinglug thicknesses as illustrated, those skilled in the art will appreciatethat the side face pairs may be parallel without departing from thescope of the present invention. As shown in FIG. 2, unlike conventionalbolts, both of the lugs 110 e and 110 f of the bolt 50 of theillustrated embodiment have tapering faces 130 e&f, 132 e&f extendingfrom its respective side face 122 e or 122 f; 124 e or 124 f to itsrespective forward face 114 e or 114 f. Although the tapering side faces130 e&f may have other dimensional characteristics without departingfrom the scope of the present invention, in one embodiment the faces aretapered at an angle in a range of about 2° to about 10.5° measured fromlongitudinal, and more particularly at an angle of about 6.25°. Althoughthe tapering side faces 132 e&f may have other dimensionalcharacteristics without departing from the scope of the presentinvention, in one embodiment the faces are tapered at an angle in arange of about 2° to about 10.5° measured from longitudinal, and moreparticularly at an angle of about 6.25°. In some embodiments such as theillustrated embodiment, the angles of the tapering side faces 130 e&fare equal to but opposite from the tapering side faces 132 e&f. Althoughthe forward faces 114 e&f may have other dimensional characteristicswithout departing from the scope of the present invention, in oneembodiment the faces are narrower than the forward faces 114 a-d,g&h bya thickness in a range of about 0.010 in. to about 0.081 in., and moreparticularly narrower than the forward faces by a thickness of about0.0455 in. Further, as shown in FIG. 4, each adjacent lug pair (i.e.,110 a, 110 b; 110 b, 110 c; etc.) is spaced by a slot 134 a-h.

As illustrated in FIG. 6, the receiver 30 also includes a barrelinterface 140 as illustrated in FIGS. 1 and 6 that interlocks with thelug portion 90 of the breech bolt assembly 42 during firing. In someembodiments, the barrel interface 140 is configured as a barrelextension. Barrel interface 140 has forward barrel-receiving end 142opposite a rearward bolt-receiving end 144. The barrel-receiving end 142has an internal threaded portion 146 configured to engage threads 148(FIG. 1) on the barrel 22 and a circumferential outer flange 150 forabutting the barrel. As shown in FIG. 7, plurality of receiver lugs 152a-h extend inward into the barrel interface 140 at the bolt-receivingend 144, and a bolt interlocking chamber 154 (FIG. 6) is provided insidethe barrel interface between the receiver lugs and the internal threadedportion 146 of the barrel interface 140. Each of the receiver lugs 152a-h has a tapered rearward guide face 158 a-h and an opposite radialforward load-bearing face 160 a-h. In addition, each adjacent receiverlug pair (i.e., 152 a, 152 b; 152 b, 152 c; etc.) is spaced by acorresponding gap 162 a-h sized and shaped for receiving correspondingbolt lugs 110 a-110 h. Longitudinally skewed cylindrical slots 164extend into gaps 162 e&f for guiding cartridges into the barrelinterface 140 from the magazine 24 as will be appreciated by thoseskilled in the art.

In operation, the breech bolt assembly 42 moves back and forth alonglongitudinal axis L as cartridges are fired from the firearm 20 during aconventional automatic or semi-automatic sequence. U.S. Pat. No.2,951,424 (Stoner), U.S. Pat. No. 3,198,076 (Stoner), and U.S. Pat. No.5,351,598 (Schuetz) describe this conventional sequence in more detail.The sequence begins with the breech bolt assembly 42 in itsrearward-most or open position (not shown). A cartridge is fed from themagazine 24 into the cartridge recess 92. Once a cartridge is positionedin the recess 92, the bolt 50 slides forward, positioning the cartridgein the firing chamber 26. As the bolt 50 moves forward, the bolt lugs110 a-h pass through the gaps 162 a-h between the receiver lugs 152 a-hof the barrel interface 140 and into the bolt interlocking chamber 154.After the bolt lugs 110 a-h pass the receiver lugs 152 a-h, the cam pin82 or another conventional means rotates the breech bolt assembly 42about the longitudinal axis L to interlock the breech in a closedposition, so the rearward faces 116 a-g of the bolt lugs 110 a-g contactthe forward load-bearing faces 160 a-g of the receiver lugs 152 a-g.

Once the breech bolt assembly 42 rotates into an interlocking closedposition with the barrel interface 140, the cartridge in the firingchamber 26 may be fired by pulling the trigger 34. Pulling the trigger24 causes it to pivot, releasing the hammer 36 and allowing the hammerto rotate from an engaged or cocked position to a disengaged position orfiring position as shown in FIG. 1. After rotating, the hammer 36strikes the firing pin 38, driving the pin forward in the bolt 50 tostrike the cartridge in the firing chamber 26, causing it to fire. Thepositions of the trigger assembly 32 and the breech bolt assembly 42just after firing are illustrated in FIG. 1.

After a cartridge is fired, the breech bolt assembly 42 rotates todisengage the bolt lugs 110 a-h from the receiver lugs 152 a-h andslides backward, extracting the spent shell casing before a newcartridge is loaded into the loading chamber 26 from the magazine 24.This process of sequentially loading, firing, and extracting continuesthrough a number of cycles as determined by the shooter.

When the cartridge fires in the firing chamber 26, it transmits a recoilforce on the bolt 50 in the direction indicated by arrow R. As aconsequence of the recoil force, bolt lugs 110 a-g are forced backwardagainst the corresponding receiver lugs 152 a-g, forming load bearingrelationships between bearing faces 116 a-g, 158 a-g of the bolt 50 andbarrel 140. As will be appreciated by those skilled in the art, therounded tapered outer faces 120 a-d&g and the tapering side faces 130e&f, 132 e&f of the bolt lugs 110 e&f prevent jamming without affectingthe surface area of the bearing faces formed between the bolt lugs 110a-g and receiver lugs 152 a-g. Thus, the bearing loads on the bolt lugs110 a-g and receiver lugs 152 a-g are unchanged from conventionaldesigns.

When the bolt 50 slides forward, the bolt lugs 110 a-h can be out ofalignment with the gaps 162 a-h between the receiver lugs 152 a-h. If aconventional bolt is used, this misalignment can prevent the bolt lugs110 a-h from passing through the receiver gaps 162 a-h, preventing thebolt from reaching to its interlocking closed position and preventingthe cartridge from being fully loaded in the firing chamber 26. Beingunable to fire, the bolt 50 will not be forced backward to its openposition, and the firearm may require servicing before it can be fired.

The rounded tapered outer faces 120 a-d&g prevent the bolt lugs 110 a-hfrom being out of alignment with the gaps 162 a-h between the receiverlugs 152 a-h. The rounded aspect of these faces 120 a-d&g tends to guidethe bolt lugs 110 a-h circumferentially so the bolt lugs and receivergaps 162 a-h are circumferentially aligned. Further, the tapered aspectof the faces 120 a-d&g tends to radially center the bolt lugs 110 a-h inthe receiver gaps 162 a-h. Thus, the rounded tapered outer faces 120a-d&g properly align the bolt lugs 110 a-h with the gaps 162 a-h andprevent jamming due to misalignment.

The tapering side faces 130 e&f, 132 e&f prevent the bolt lugs 110 e&ffrom circumferentially interfering with the corresponding receiver lugs152 e-g, even when the bolt lugs 110 a-h are circumferentiallymisaligned with the receiver gaps 162 a-h. Further, the forward faces114 e&f of the bolt lugs 110 e&f are of conventional height so the boltlugs can push cartridges along the longitudinally skewed slots 164 inthe barrel interface 140 to fully seat in the firing chamber 26.Further, as will be appreciated by those skilled in the art, the skewedslots 164 in the barrel interface 140 tend to circumferentially andradially center the bolt lugs 110 e&f as the bolt 150 is driven forward.

Preferably, components of the breech bolt assembly 42 and the barrelinterface 140 are manufactured from a metal suitable for use in firearmsusing techniques known to those skilled in the art. Furthermore, it ispreferred that the bolt 50 and barrel interface 140 each be formed froma single, unitary piece of material; however, in alternativeembodiments, the bolt and the barrel interface may each be made bycoupling two or more separate components as would occur to one skilledin the art. Also, it is contemplated that the bolt assembly 42, the bolt50, and the barrel interface 140 may be formed from different materialssuitable for their intended purpose.

As used herein, the term firearm is intended to include rifles,shotguns, pistols, and other portable guns. Further, the term firearm isnot intended to be limited to center-fire weapons as described above.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference.

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims. Although specificlanguage is used to describe features of the illustrated embodiment, itshould be understood that no limitation of the scope of the invention isthereby intended. Any alterations and further modifications in thedescribed device, and any further applications of the principles of theinvention as described herein are contemplated as would normally occurto one skilled in the art(s) to which the invention relates.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the”, and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including”, and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions, products,and methods without departing from the scope of the invention, it isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:
 1. A breech bolt assembly for mounting in a firearmfor loading a cartridge in a firing chamber of the firearm prior tofiring a projectile from the cartridge and extracting a shell casingfrom the firing chamber after firing, the bolt assembly comprising: abolt including an elongate cylindrical body portion having alongitudinal centerline, a lug portion extending forward from the bodyportion, and a firing pin bore extending through the bolt substantiallyparallel to the longitudinal centerline, said lug portion including aplurality of spaced lugs extending outward therefrom, at least three ofsaid spaced lugs being of substantially equal height and equally spaced,a cartridge recess at a front end sized and shaped for holding arearward end of the cartridge therein, an extractor cavity extendingrearward from the cartridge recess, and an off-center longitudinal holeat least partially aligned with the cartridge recess; an extractorpivotally mounted in the extractor cavity and biased to hold thecartridge in the cartridge recess; and an ejector pin slidably receivedin the off-center longitudinal hole, the pin being biased to eject theshell casing from the cartridge recess; wherein at least two of theplurality of lugs on the lug portion of the bolt have: a flat topsurface portion extending from a rearward face of the respective lugs,the rearward face having a quadrilateral shaped perimeter with a curvedbottom edge, an arc of the curve extending toward a top edge of therearward face, the flat top surface portion having an arcuate leadingedge; and a rounded tapering outer face extending from the arcuateleading edge of the flat top surface portion along more than 25% of alongitudinal length of the respective lug to a forward face of therespective lug, the forward face having a rounded top edge, the rearwardface of the respective lug being taller than the forward face of therespective lug; and wherein the rounded tapering outer face of eachrespective lug has an outer radius that is less than a radial distancebetween said centerline and the corresponding outer face.
 2. The breechbolt assembly as set forth in claim 1, wherein each of the taperingouter faces is tapered at an angle in a range of about 10° to about 32°measured from longitudinal.
 3. The breech bolt assembly as set forth inclaim 2, wherein the angle at which each of the tapering outer facestapers is about 21° measured from longitudinal.
 4. The breech boltassembly as set forth in claim 1, wherein the forward face of each lughaving the tapering outer face is shorter than the correspondingrearward face by a radial distance in a range of about 0.025 inches toabout 0.045 inches.
 5. The breech bolt assembly as set forth in claim 4,wherein the radial distance by which the forward face is shorter thanthe rearward face is about 0.035 inches.
 6. The breech bolt assembly asset forth in claim 1, wherein at least one of the plurality of lugs onthe lug portion of the bolt without a tapering outer face have taperingside faces so that the rearward face of the respective lug is wider thanthe forward face of the respective lug.
 7. The breech bolt assembly asset forth in claim 6, wherein each of the tapering side faces tapers atan angle in a range of about 2° to about 10.25° measured fromlongitudinal.
 8. The breech bolt assembly as set forth in claim 7,wherein the angle at which each of the tapering side faces tapers isabout 6.25° measured from longitudinal.
 9. The breech bolt assembly asset forth in claim 6, the forward face is narrower than the rearwardface by a distance in a range of about 0.010 inches to about 0.081inches.
 10. The breech bolt assembly as set forth in claim 9, thedistance by which the forward face is narrower than the rearward face isabout 0.0455 inches.